1. Field of the Invention
The present invention relates to frequency stabilizing equipment suitably applicable to a light source used in wavelength division multiplexing (hereinafter referred to as xe2x80x9cWDMxe2x80x9d) optical communication and more particularly to the frequency stabilizing equipment for improving accuracy and stability of a frequency of light from the light source employed in optical communication.
2. Description of the Related Art
Recently, an increase in transmission amounts is required for optical communication. To meet the requirement of increased amounts of light signal transmission, conventionally measures of space division multiplexing that can increase logarithmic values of optical fibers and of multiplexing in electric fields including frequency division multiplexing (FMD) and/or time division multiplexing (TDM) have been employed. However, since these measures cannot fully satisfy such requirements of rapid increases in transmission amounts, development of the WDM optical communication technology is being pursued in recent years. In the WDM optical communication technology, by using light from the WDM light sources having different wavelengths, a plurality of independent transmission paths can be constructed in one optical fiber, providing increased amounts of transmission for optical communication. However, to use this technology successfully, improvement of accuracy and stability in a frequency of light used as light from the WDM light source is essential.
As a means to achieve this object, an advanced technology has been proposed, for example, in a literature xe2x80x9cLight source frequency stabilizing characteristics obtained by using a Mach-Zehnder filter in site environmentxe2x80x9d (Sakamoto and Oda: NTT, Society of Communication B-10-216, 1997). In the proposed technology, light of a plurality of WDM light source frequencies is provided so as to be fixed to peak points of light having a plurality of transmission frequencies generated by a Mach-Zehnder filter.
However, this technology has the following problems to be further solved.
1. Though it is theoretically true that, by using the Mach-Zehnder filter, both transmission frequency bands and rejection frequency bands are allowed to occur in a comb-teeth like manner at intervals of an equal frequency, when exacting specifications are required, a deviation from targeted frequency caused by production errors and/or polarization dependency and other factors may occur. Therefore, there is a possibility that a deviation occurs between the peak point of the light having a plurality of transmission frequencies and a point where the light having a plurality of light source frequencies appears.
2. It is impossible to freely select the light source frequency due to a restraint by the occurrence of comb-teeth like light waves having transmission frequencies generated by the Mach-Zehnder filter.
3. To guide light having the WDM light source frequencies to the peak point of the light waves having a plurality of transmission frequencies generated by the Mach-Zehnder filer, it is necessary to perform dithering by applying a sinusoidal power source to a heater power source of the filter (the dithering represents an operation in which the characteristic of light appearing in a comb-teeth manner induced by the Mach-Zehnder filter are changed). Therefore, there are some cases where considerations must be given to changes in temperature that may affect peripheral devices.
The present inventors have proposed a technology in U.S. Ser. No. 09/257,369 to solve these problems. In the application, frequency stabilizing equipment for a light source is disclosed which is comprised of a light source having a control circuit adapted to adjust a frequency of carrier light for optical communication, a light modulator to modulate intensity of carrier light outputted from the light source by using a marker signal, a delay generator to provide time delay to the carrier light to be modulated based on wavelengths of the carrier light modulated by the modulator, a photoelectric converter to convert the time delay given by the delay generator to an electric signal, a phase difference detector to detect the time delay in accordance with the electric signal fed by the photoelectric converter and a control signal generator to generate a control signal to be used in the control circuit in the light source based on a detection signal fed from the phase difference detector.
The control circuit in the light source employed in the frequency stabilizing equipment is adapted to control so that the carrier light from the light source has a predetermined frequency based on a phase difference between the marker signal used as a modulation signal and carrier light to be modulated to which the time delay has been provided by the delay generator and the provided time delay has been converted to the electric signal. This enables a variation in frequencies of the carrier light for optical communication to be suitably prevented without the use of the Mach-Zehnder filter. Therefore, by using the technology disclosed in the above application by the present inventors, the shortcoming described above can be solved and preferable carrier light for optical communication can be obtained without using the Mach-Zehnder filter.
In view of the above, it is an object of the present invention to provide improved frequency stabilizing equipment for a light source for optical communication.
According to a first aspect of the present invention, there is provided frequency stabilizing equipment for optical communication light source comprising:
a light source to output carrier light for optical communication, having a control circuit being able to adjust a frequency of the carrier light;
a modulator to perform light strength modulation of the carrier light outputted from the light source by using a marker signal;
a delay generator to provide time delay to the modulated carrier light depending on a wavelength of the modulated carrier light modulated by the modulator;
a photoelectric converter to convert the modulated carrier light including the time delay provided by the delay generator to an electric signal;
a phase difference detector to detect the time delay based on the electrical signal fed from the photoelectric converter;
a control signal generating device to generate a control signal used for the control circuit based on a detected signal fed from the phase difference detector in order to achieve stabilization of a frequency of the carrier light from the light source; and
whereby the photoelectric converter is comprised of a local oscillating light source in order to produce a local oscillating light signal having a wavelength being different from that of the carrier light, an optical coupling device to bind the local oscillating light signal fed from the local oscillating light source to the modulated carrier light from the delay generator in order to optically amplify the modulated carrier light from the delay generator and a light receiver having a non-linearity characteristic between an input signal and output signal to receive a light signal from the coupling device and to output an electrical signal amplified non-linearly to the phase difference detector.
In the foregoing, a preferable mode is one wherein the local oscillating light source is given a feedback control to maintain a predetermined light intensity.
Also, a preferable mode is one wherein the optical coupling device is a coupler.
Also, a preferable mode is one wherein the delay generator is a dispersion generator.
Also, a preferable mode is one wherein the marker signal is a modulated signal having a predetermined angular velocity.
According to a second aspect of the present invention, there is provided frequency stabilizing equipment for optical communication light source comprising;
a light source to output carrier light for optical communication, having a control circuit being able to adjust a frequency of said carrier light;
a modulator to perform light strength modulation of the carrier light outputted from the light source by using a marker signal;
a delay generator to provide time delay to the modulated carrier light depending on a wavelength of the modulated carrier light modulated by the modulator;
a phase difference detector to detect time delay of the modulated carrier light provided by the delay generator;
a control signal generating device to generate a control signal used for the control circuit based on a detected signal fed from the phase difference detector in order to achieve stabilization of a frequency of the carrier light from the light source; and
whereby the phase difference detector is comprised of a branching circuit to divide an output signal fed from the delay generator into two output signals and to provide the two output signals a difference in phase of xcfx80/2 to each other, a pair of integrating circuits to multiply the output signals outputted from the branching circuit by the marker signal respectively, a pair of low-pass filters to take out low frequency components of electrical signals fed by the integrating circuits and an arithmetic unit to take out phase difference components between the marker signal and the modulated carrier light to be fed by the delay generator by means of dividing the low frequency components obtained from one of the pair of filters by the low frequency components obtained from the other of said pair of filters.
In the foregoing, it is preferable that the arithmetic unit is used to calculate phase difference signals sin xcex8 or cos xcex8 by processing arithmetic operations using a formula:
F=(XL2+YL2)1/2
which is obtained from values of phase difference components XL and YL and where an output signal from one filter is represented as XL=F sin xcex81 and an output signal from the other filter is represented as YL cos xcex81.
Also, it is preferable that the frequency stabilizing equipment further comprises a photoelectric converter to convert the modulated carrier light provided by the delay generator into an electrical signal and to output the electrical signal to the phase difference detector, whereby the photoelectric converter is comprised of a local oscillating light source to produce a local oscillating signal having a wavelength being different from that of the carrier light, an optical coupling device to bind the local oscillating light signal fed from the local oscillating light source to the modulated carrier light from the delay generator in order to optically amplify the modulated carrier light from the delay generator and a light receiver having a non-linearity characteristic between an input signal and output signal to receive a light signal from the coupling device and to output an electrical signal amplified non-linearly to the phase difference detector.
Also, it is preferable that the local oscillating light source is given a feedback control to maintain a predetermined light intensity.
Also, it is preferable that the optical coupling device is a coupler.
Also, it is preferable that the delay generator is a dispersion generator.
Also, it is preferable that the marker signal is a modulated signal having a predetermined angular velocity.